1. Field of the Invention
The present invention relates to aerodynamic devices or wings typically used to propel boats, vehicles or persons across water, land or ice or snow, to propel airborne craft above ground or in space, or to propel nano-technological devices. More specifically this invention relates to an improved aerodynamic device and method of using the same designed to replace specialized downwind and cross wind sails on sailing boats sometimes referred to as spinnakers. The improved aerodynamic device and method of the present invention is applicable to mono-hulled, and multi-hulled boats in a variety of tonnages.
2. Description of the Related Art
A search of the prior art located the following United States patents which are believed to be representative of the present state of the prior art: U.S. Patent Publication No. US 2003/0140835A1, published Jul. 31, 2003, U.S. Pat. No. 5,355,817, issued Oct. 18, 1994, U.S. Pat. No. 5,642,683, issued Jul. 1, 1997, U.S. Pat. No. 4,497,272, issued Feb. 5, 1985, U.S. Pat. No. 5,033,698, issued Jul. 23, 1991, U.S. Pat. No. 4,708,078, issued Nov. 24, 1987, U.S. Pat. No. 4,296,704, issued Oct. 27, 1981, U.S. Pat. No. 4,129,272, issued Dec. 12, 1978, U.S. Pat. No. 3,720,180, issued Mar. 13, 1973, and U.S. Pat. No. 3,356,059, issued Dec. 5, 1967.
While the following discussion focuses on recreational sailcraft applications, the example is not intended to be limiting. The device of this invention may be used in other flying situations and also with other types of vessels, vehicles and marine structures.
While using kites, parachutes and other kite-like devices to draw sailing vehicles is not a new enterprise, to date most existing sailcraft utilize a fundamentally imbalanced force couple when connecting their wind-effected elements to their water-effected elements. This imbalance causes a lateral overturning moment, which most sailing craft counter either by the addition of ballast below the vessel's center of buoyancy, or by offsetting significant structure athwartships, or both. Either solution offsets the boat's center of gravity and center of buoyancy as the boat heels, offering a countering, or righting, moment. In addition to cost, weight and drag increases associated with either solution, a stability limit is eventually reached whereby no more sail area, or no greater wind speed may be utilized. A kite's entire force may be attached to the boat at the deck, or even near or below the waterline, reducing the overturning moment arm to near zero, ameliorating much or all of the overturning torque at its source, thus obviating the need for traditional solutions.
A corollary to the above relates to longitudinal overturning moment. A typical sailcraft develops a significant longitudinal or pitching moment as well, again due to the sails' resultant force being vertically offset from the hull's drag forces. This is particularly evident in powerful and fast racing craft, and is typically countered by increasing the buoyancy of the craft's hull(s) forward, shifting weight aft, or by de-powering the sailing rig at top speed, in order to remain within the vehicle's performance envelope. Bringing a kite's attachment point down to the deck or below reduces or eliminates this pitching moment, allowing the vessel to safely be sailed harder at higher speeds. It also allows the designer to contemplate hull shapes, equipment layouts and weight trimming schemes which do not need to counter this said pitching or overturning moments, but can maximize the vessel's speed, ride and/or cost savings instead. Such a vessel can carry ordinary sized kites in appreciably higher winds, or appreciably larger kites in ordinary winds. Alternatively, such a vessel can be designed with far less inherent stability, or utilizing significantly decreased weight, cost, and/or complexity. Such considerations may allow new classes of sailing boats to sail significantly faster, safer and/or more comfortably than existing sailing vessels.
The use of kites also allows sailing rigs to be placed on non-sailing craft or other mobile marine or vehicular structures without regard for whether or not such boat or structure has sufficient transverse stability to carry a conventional sailing rig. For example, commercial vessels, cargo ships, oil drilling rigs and barges of all types may be self-powered with kites, without significant alteration to their structure.
If fitted in lieu of the sailboat's mast and all rigging, a kite can reduce a vessel's all-up weight significantly; also her all-up windage, cost and structural complexity. Since ordinary sailing vessels rely on highly tensioned shrouds and stays, taken to the vessel's extremities, and commensurately highly compressed mast and mast mounting structures, with all forces dissipated throughout the vessel's structure, such structure must be engineered to withstand such forces, in addition to forces associated with payload, sea state and prudent safety margins. When replaced by a kite, there are only pure traction forces which can be concentrated at a single attachment point or small attachment area in the central part of the vessel, and all non-traction rigging forces can be eliminated completely, leaving the designer or structural engineer with much smaller, centralized forces to dissipate through the structure. This may lead to lighter, stronger, faster and less expensive racing boats, and also to simpler, lighter, less expensive retrofitting of kites to existing sailing and non-sailing boats, ships and marine structures.
Flying a boat's wind-effected structure(s) at altitudes higher than a boat's typical mast and sail can yield more wind energy than current practice harvests. Friction and turbulence with the water's surface slows surface winds appreciably for some distance above the water's surface. Meteorological studies estimate approximately as much as 20–30 percent higher velocity to wind at 100–150 feet above the surface than at 10–20 feet, for a range of typical wind velocities and sea states. Energy in the wind varies with the square of the wind's velocity, a 30 percent increase in velocity yields a potential 69 percent increase in available energy, if the vessel's sails can be deployed in such higher velocity stream. Simply flying an identically-sized kite at higher altitude than a competing racing yacht's sail, for instance, can yield a race-winning advantage through the greater energy available.
Since a free flying kite is functionally decoupled from the boat it is pulling, it is free to accelerate and/or to fly at speeds different from those the hull experiences. For instance, if a kite begins high in the sky, and is then dived towards the water at the same time the hull begins to accelerate, the kite will reach a high percentage of the wind's speed (often greater than unity) far sooner than will the hull. The apparent wind experienced by the kite will be much greater than that experienced by the boat. If sailed alongside an identical boat with identically-sized sails, such a maneuver will yield faster acceleration of the kite-driven hull than of the conventionally driven one.
Similarly, if the kite is flown in a constant zig-zag or sine wave pattern, it may fly at all times faster than the hull and will experience continually higher apparent wind than either its own hull or the hull and sails of similar competitors nearby. It will thus harvest and deliver commensurately more power to its attached hull, yielding higher speed than its competitors. At the same time, since the hull does not travel at the kite's speed, the entire vessel does not suffer appreciably increased drag penalties due to increased aerodynamic drag on the hull, crew and associated appendages.
Conversely, using such maneuvering of the kite, or “dynamic sheeting” to increase power, a kite powered vessel may develop similar power and boat speeds as other similarly sized vessels while using significantly smaller kites than the other boats' sails. This might yield significant cost savings, weight savings, or allow the effected boat to use smaller sail handling equipment, fewer crew, smaller control energy inputs, etc. Such downsizing can result in a “beneficial spiral,” whereby utilizing smaller lighter equipment and crews allows smaller, lighter hulls to be used, resulting in the need for even smaller, lighter kites, lighter control equipment and crew, etc.
Flying the kite decoupled from the boat, at altitudes substantially above the water's surface can generate lower transient or shock stresses on the kite structure. Motions of the boat, and especially of its masthead and other extremities when sailing in rough water largely aren't transferred to the kite structure. Perturbations of the wind's flow caused by friction and interference with surface discontinuities (water waves) which adversely effect normal sails largely do not effect kites. As a result of these factors, airflow across the kite is smoother and aerodynamic effects are largely not interrupted. In addition, since the kite structure does not experience large and sudden accelerations due to such motions, either at its attachment points or throughout its structure, building of materials which are light weight, of high modulus and/or very limited in stretch characteristics may be contemplated. These materials are contra-indicated in ordinary spinnakers whose shock loads often exceed the mechanical properties even of superior materials.
It should be noted that the simultaneous use of all of the above: non-heeling, lighter or non-existent masts and sail handling equipment, higher winds aloft and dynamic maneuvering of the kite can create a synergistic effect, pushing the “beneficial spiral” ever farther.
Prior art in the field teaches kites and kite-like devices which have limitations for use aboard sailing vessels. Some are difficult to deploy or fly in close proximity to boat hulls. Others are difficult or dangerous to assemble, launch or recover in high winds. Others have low wind thresholds, or stall speeds, which are too high to make them useful in light winds. Still others are expensive to produce or difficult or expensive to maintain and repair. Others require specialized equipment to launch, control or recover. Still others have features or physical characteristics which render them not rule-legal for yacht racing under widely accepted yacht racing rules and definitions (e.g., the International Sailing Federation, “ISAF”).
Launch and recovery techniques and equipment have been investigated by others. Various levels of complication, reliability and expense have been proposed, from pre-launched leader or pilot kites to air cannons to hydraulically extendable masts to the use of lighter than air gas-filled balloons, aerostats and other shapes as kite launch assistors. A system which is simple, reliable and fits within sailors' existing skill-sets has eluded invention to date.
Kite designs may be broadly characterized according to their construction. The range of constructions covers a broad spectrum from framed, semi-framed or unframed single skin kites to double skinned, air filled or “ram air” type kites, to hybrid combinations of these various types. Regardless of their construction type, all kites must have some means of maintaining their chord wise and span wise shape while flying.
Several ways of forming or contributing to the form of the profile and chord wise and span wise shape of a wing while it is flying are known and practiced in the art.
One approach is the use of kites which have little or no aerodynamic refinement. Profiles which resemble hemispherical parachute canopies have been proposed, including various methods to control both position and power of such kites. See, for example, Bedford (U.S. Pat. No. 5,642,683) or Stanford (U.S. Pat. No. 4,722,497) and also toy-like devices consisting of flat rectangular pieces of cloth with lines attached to the 4 corners.
Other apparatus and methods use rigid or semi-rigid frame assemblies. See for example Roeseler (U.S. Pat. No. 5,366,182).
Another approach is to build non framed, air filled wing shaped structures and to contrive for air pressurization by being open near a flow stagnation point to bleed into internal spaces within the wing, which pressurized air then functions as a structural element. This is often referred to as the ram air system after Jalbert (see for example U.S. Pat. No. 3,285,546).
Yet another approach is to arrange some elements of the wing's aerodynamic surfaces to be otherwise than perpendicular to the general axis of the flying lines and set so as to generate aerodynamic forces that cause the wing tips to pull away from each other. See, for example, Lynn (U.S. Patent Publication No. 2003/0132348).
In another approach, multiple bridle lines may be attached to the wing at intervals span wise and chord wise and these bridle lines converge to the flying lines at a point or points between the operator and the wing. Such bridles, by distributing the tension in the flying lines more evenly over the surface of the wing, reduce the bending load on span wise structural elements and therefore assisting in the retention of span wise form. See, for example, Schimmelpfennig (U.S. Pat. No. 5,033,698)
In the case of kites with rigid or semi-rigid frames, multiple bridles or else triangular or quadrilateral shaped pieces of material attached at one edge and standing out from the kite's surface called keels or flares make it possible for these frame elements to be proportionally smaller and lighter, however both the weight of the frame and the air drag of multiple bridle lines or keels are detrimental to cost, aerodynamic efficiency and manufacturing simplicity.
Limitations of Current Art
In the case of simple soft flat or hemispherical-shaped kites the major drawback for sailing use revolves around two issues; multiple bridle lines and aerodynamic efficiency. Many-bridled kites are difficult to launch from the crowded and complex confines of typical boat decks. Kites with low aerodynamic efficiencies (Lift/Drag (“L/D”)≦1) can only be used for a narrow range of sailing courses, perhaps 20–40 degrees either side of dead downwind. Playsail-type kites (for example, see: http://www.nyke.org/Play Sail Workshop.htm) need a large separation of their flying anchors and suffer leading edge collapse upon acceleration, thus are not presently used in sailing applications.
In the case of conventional Jalbert, parafoil or parapent style foil kites that use ram air inflation as their structural element, the pressure differential available is so small as to allow no possibility of sufficient span wise beam strength without support from multiple bridles spaced at intervals both chord wise and span wise. Because more bridles allow thinner and more aerodynamically efficient airfoil sections to be used and also permit higher aspect ratio form thereby reducing induced drag there has been a tendency in recent years for parafoil style traction kites to have upwards of 60 bridle lines.
In the case where aerodynamic forces are used to retain or assist in the retention of span wise form, multiple bridles typically reduce the proportion of the kite's aerodynamic surfaces that are required to be other than approximately perpendicular to the flying lines and hence increase the proportion of surface area that can be applied directly to supplying pull on the flying lines. An advantageous consequence of this can be a higher lift coefficient, which manifests as more pull in proportion to overall size. Bridle lines do however, in themselves, add undesirable drag and often tangle during launching from deck or during flying in such a way as to prevent satisfactory operation of the kite.
Traction kites using various combinations of these contributions to chord wise and to span wise shape are known and used. Each have inherent advantages and disadvantages pertaining to cost, tangle resistance, luff resistance, specific power, upwind ability, packing ease, relaunch ease (especially from water), gust responsiveness and other values.
Examples of rigid or semi rigid framed kites include Allison (U.S. Pat. No.2,737,360) and Roeseler (U.S. Pat. No. 5,366,182). These kite types rely largely on the cantilever strength of their struts or frames for their aerodynamic shape. These kites are typically heavier than non-framed kites and are not amenable to scaling to large sizes, due to physical scaling factors increasing their specific weight (weight per unit area) unfavorably.
An example of a traction kite using mainly a combination of multiple. bridle lines and aerodynamic forces to hold span wise form is described in WO99/59866. This kite has a very flexible spar or bundle of spars comprising the leading edge of the kite and multiple panels, separated from each other by sets of bridles arranged in the flow wise direction and with these panels arranged and shaped so that the aerodynamic pressure distribution around them provides the major contribution to the span wise form for the kite. This kite is still heavier than frameless kites and, as it partially relies on its rigid framework, still suffers from scaling factors at large sizes. In addition, both these and framed kites are typically characterized by sharp leading edges, which limit the kite's ability to respond efficiently to varying or extreme angles of attack.
An example of a traction kite using a combination of only aerodynamic forces and multiple bridles to hold its span wise and chord wise form and with no rigid, semi-rigid or ram air structural elements is what has become known as the NASA parawing or NPW (e.g., http://www.npw5.com/). A bridle line is a line which does not run uninterrupted from the kite wing surface to the ultimate end of attachment, but rather joins together with other such lines, typically in a grouped or cascaded manner. These grouped bridle lines typically move as a group when their controlling attached “flying line” is manipulated by the kite forces or the handler. The NASA para-wing traction kite has a single skin, shaped and supported by bridles in such a way as to generate aerodynamic forces that are sufficient to form the span wise and chord wise shape of the kite. NASA wings are themselves ramifications of earlier wings after Rogallo (See U.S. Pat. Nos. 2,546,078 and 2,751,172). This type of kite is light in weight as it is both frameless and is built of a single layer of structural material, but it suffers drag penalties associated with its multiple bridle lines (typically 30–50 lines) and its generally non-smooth surface shape. In addition, multiple-bridled kites suffer from tangling issues during launch and recovery operations. Also, as many of the bridle lines are taken to the kite's interior surface, strong tearing and peeling loads are created throughout the structure, which make local reinforcing necessary and scaling problematic.
Also see an improved form of NASA wing known as ESB (http://home.swipnet.se/telsplace/ESP12/ESPMod2.Html) and also (http://home.swipnet.se/telsplace/Tug ESP3/ESPMod3.html which has a more aerodynamically efficient shape and reduced number of bridle lines, but remains in a state of development pending improvements in stability, controllability and launch from boat decks.
An example of a traction kite using ram air inflation as its span wise structural element in combination with multiple bridle lines is described in Schimmelpfennig (U.S. Pat. No. 5,033,698). This kite is a double skin ram air inflated envelope with airfoil profile ribs, without any frame but with multiple bridles distributed chord wise and span wise over the surface of the kite, and primarily near to the kite's leading edge, to assist retention of chord wise and span wise form. It uses aerodynamic forces to reduce the number of chord wise bridle lines, but it does not use significant contribution from aerodynamic forces to maintain its span wise shape. These kites are typically heavier and more expensive than single-skinned frameless kites, yet lighter than rigid or semi-rigid framed kites. They suffer from minimal available inflation pressure, necessitating either thicker than optimal airfoil profiles, the need for multiple bridle lines with their attendant tangling and drag problems, or both.
An example of a traction kite using ram air inflation in combination with a span wise semi-rigid structural element and some contribution to span wise form from aerodynamic forces but without multiple bridling is described in Jones, et. al. (U.S. Pat. No. 4,363,458). This kite is a double skin ram air inflated airfoil with a semi-rigid spar spanning its leading edge. Only two flying lines are used, one attached to each wing tip at the leading edge. The trailing edge of the kite is unsupported by bridles or flying lines. This kite solves the multiple bridle issue, but substitutes unwanted added weight of a substantial cantilevered solid spar at its leading edge, and also suffers from scaling issues.
An example of a traction kite using a semi-rigid frame in combination with a single or double skin and some use of aerodynamic forces but without multiple bridles or at least with very few bridles is described for instance in Legaignoux (U.S. Pat. No. 4,708,078). This kite has an armature of inflated tubes covered by one or two flexible skins. The tubes can be inflated through one or more sealable orifices using a pump for example to pressures significantly greater than would be available by ram air inflation. These tubes form a semi-rigid spar along the leading edge of the kite and usually also a series of chord wise orientated spines extending from the leading edge to the trailing edge at intervals across the span of the kite. Heavier than other single skin kites and heavier than even most double skin frameless kites, this kite may be categorized as a semi-rigid frameworked kite as it relies ultimately on the cantilever strength of its inflated tubes for its ability to maintain its aerodynamic shape in sizes larger than those used for small sailboards and the like. The tubes must be of sufficiently large diameter and remain sufficiently inflated to maintain the shape required for the kite to fly satisfactorily. This kite thus suffers similar scaling issues to framed kites previously cited.
Examples of kites which use primarily aerodynamic forces to maintain their shape, with or without substantial ram air inflation are for example those of Barish (U.S. Pat. Nos. 3,298,635 and 3,558,087) and Lynn (U.S. Patent Publication No. 2003/0132348). These kites either have substantially a single layer skin (Barish) or minimal bridle lines (Lynn), but each suffers from one or more of the drawbacks of other kites cited above, i.e., high weight, peel or tearing forces, manufacturing complexity, line tangling or scaling limitations.
No existing kite or kite type in any combination of the above attributes or any others, prior to the preferred embodiment of this invention, are currently capable of being ruled as legal spinnaker sails for sailcraft racing, for instance under existing International Sailing Federation (ISAF) 2002 Yacht Racing rules. Such yacht racing rules specifically prohibit double skins or interior structure, inflated components, ram air, framing or supporting materials of any kind, prohibit openings into or holes through any part of the structure or flares or other surface discontinuities and prohibit more than three total flying lines, each lead directly to the corners of a substantially triangular piece of foldable, flexible material.
It is therefore an object of this invention to develop a class of kite-like structures capable of flying freely while towing a vehicle, supporting itself in stable, controllable tethered flight at L/D ratios in excess of two. The structure and the vehicle it powers therefore benefit from all the attributes of sailing kites as cited earlier.
It is a further object of this invention to achieve the above in a class of structure which have only a single layer of foldable, flexible, lightweight material, for instance nylon spinnaker cloth, without pressure inflated, ram air inflated or rigid or semi rigid struts or appendages of any kind.
It is yet a further object of the present invention to achieve the above without the use of multiple bridle lines, but to do so with as few as two, three or four total flying lines, each taken directly to the kite structure.
It is yet another objective of the present invention to achieve the above by creating a class of shapes in which all aerodynamic and tether line forces devolve into the kite's structure substantially as tension-only forces, thus largely eliminating compressive, cantilever, tear or peel forces from the (typically) fabric structure, allowing scaling of the device to relatively very large sizes without undue deleterious scale effects. The class of structure should use only aerodynamic forces to maintain its flying shape.
Yet another object of the present invention is to create a class of kite-like devices which may be easily and familiarly launched, flown and recovered from small and large vessels while afloat without assistance and with or without specialized sail-handling equipment, by sailors with typical expertise and strength.
Still another object of the present invention is to achieve all of the above while resulting in a class of devices which are efficient to manufacture, easy to maintain and to repair, inexpensive to transport and that “look and feel” similar to existing spinnaker sails so as to be readily acceptable by amateur and professional sailors of average ability.
It is a further object of the present invention to create a class of flying structures which hew specifically to the legal definition for “spinnakers” as defined in the International Sailing Federation (ISAF) 2002 yacht racing rules and/or other similar yacht racing rules.
It is still further an object of the present invention to provide for the simple addition of lighter than air (LTA) gasses to the class of structure, with minimal alteration or aerodynamic effect, in order to effectuate extremely light wind performance, launch and retrieval.